A wellbore may penetrate various reservoirs, intervals or other zones of interest. In some instances, the length or extent of a wellbore may render it impractical to apply a single treatment to the complete wellbore. For example, a single stimulation operation may be problematic in subterranean formations comprising multiple production zones along the wellbore.
Stimulation operations, such as hydraulic fracturing, acidizing, fracture acidizing, or hydrajetting, are usually performed after the drilling and completion of a wellbore to enhance hydrocarbon production (e.g., oil, gas, etc.) into the wellbore. There are usually multiple zones along a wellbore from which it is desirable to produce hydrocarbons. Different zones tend to have different fracture gradients. Moreover, in a situation wherein some zone along a wellbore is depleted, depletion generally lowers the fracture gradient of the zone. When a stimulation operation is simultaneously conducted on more than one production zone, the stimulation treatment will tend to follow the path of least resistance and to preferentially enter the most depleted zones. Thus a single stimulation operation may not achieve desirable results in those production zones having relatively higher fracture gradients. Therefore, it may be desirable to perform numerous treatments in a number of zones along the length of the wellbore (including both vertical wellbores and horizontal wellbores). In some situation, it may be desirable to perform numerous treatments to different zones of a subterranean zone penetrated by multiple horizontal wellbore constructions where the subterranean zone is intersected by one or more horizontal wellbores extending outwardly from one or more parent wellbores.
One method to isolate a zone of interest and to provide for separate treatments of multiple zones in treatment of a wellbore is to use one or more sand plugs in the wellbore. In a multi-stage fracturing treatment of a vertical wellbore, the wellbore may be perforated through a first, lower zone of interest. A fracturing fluid may then be injected into the vertical wellbore to fracture the formation through the treatment perforations. After the fracturing is completed, a sand plug may be positioned across the treatment perforations. Thereafter, a formation above the sand plug may be perforated and fractured by the same technique. By the use of sand plugs of a variety of depths, a plurality of formations in a vertical wellbore may be fractured and treated independently of one another. However, when the wellbore to be treated is a horizontal wellbore, sand plugs are often difficult to be set in a manner that makes them reliable. For example, in a horizontal wellbore, a sand plug may slump and fail to fully plug the diameter of the wellbore, i.e., the sand plug may not have sufficient height to create a bridge across the wellbore or casing in the wellbore.
Removal of sand plugs in a horizontal wellbore after an isolation stage completes can also be an issue. While a clean-up procedure or a drill-out procedure can be employed to remove the sand plug, it often entails considerable time and expense to carry out. Hence improvements in the sand plug systems, in particular those used in a substantially horizontal wellbore, are still desirable.